As shown in FIG. 1, gas turbine engines 10 of the type commonly found on many aircraft include a compressor 20, a combuster 30 and a turbine 40. The compressor 20 compresses air which is then mixed with fuel for combustor 30 to ignite. The combustor 30 exhausts gases which turn the vanes of the turbine(s) 40. Power from the rotating turbine 40 operates the compressor 20.
Turbine engine compressors can be designed to supply more compressed heated air than is needed to operate the engine 10. This additional compressed air from the compressor 20 can be used for tasks other than feeding the combustor 30. For example, it is common to bleed some of the compressed air from the compressor 20 and route the bleed air to other equipment onboard the aircraft such as de-icers, cabin pressurization systems and the like.
While it is highly useful to bleed hot air from the gas turbine engines for use for other purposes, it is also important to ensure that the operation of the engines is not compromised in any way as a result of the bleed air system. If the bleed air system develops a leak, hot air bled away from the engines could escape and damage the aircraft structure. Alternatively, leakage of hot air from the bleed air duct onto other sensitive components nearby could damage those other components. Because aircraft are constantly in motion and are subjected to stresses and strains from landings, takeoffs and in flight turbulence, it is desirable to monitor the condition of bleed air ducts to ensure that no leakage is occurring.
One common way to detect leakage from a bleed air duct is by monitoring the temperature on or near the duct. Thus, some aircraft include bleed leakage detection systems to detect when leakage from the bleed air system occurs. Because the bleed air is heated by the gas turbine engines, it is common to use temperature sensing to detect bleed air leakage.
Various types of temperature sensors including thermal switches have been used in the past. A common form of thermal switch is the simple thermostat of the type that controls central heating in many homes throughout the world. Such a thermal switch consists of two metal strips having different thermal expansion coefficients. Such well known bimetallic strips flex or bend in response to temperature changes. When temperature exceeds a predetermined level, thermal expansion between the two different metals provides enough bending force to cause the bimetal strip to move close electrical contacts. Such thermal switches are highly reliable and can be used in bleed air leakage detection systems.
For example, it is possible to install thermostats near each point where the bleed air duct is joined with another section of duct work. If leakage develops, the air surrounding the ductwork joint increases in temperature and the resulting heating provides sufficient energy to actuate the thermostatic thermal switch.
One type of prior art bleed leakage detection system is referred to as the “Overheat Detection System” (“ODS”). In such system, a cable externally installed in parallel with the bleed duct includes two resistive wires that are immersed in a special salt solution. When leakage occurs, the temperature outside the duct rises and the heating causes the equivalent conductance between the two wires to decrease significantly. This change in conductance is detected by electronics (e.g., a microprocessor) which closes the bleed valve and provides a “duct leakage” message to the pilot's control panel. Because the equivalent conductance depends on the length of the cable, this Overheat Detection System is able to detect the exact place where the leakage is occurring. Unfortunately, the Overheat Detection System is a relatively expensive and complex system.
Another known prior art overheat detection system uses sensors to detect breaks in the duct-work of the bleed system. The sensors comprise surface sensors consisting of cylindrical wires a few millimeters thick that contain, between the core and sheath, a filling that has a temperature-dependent electrical resistance. Below a certain response temperature (which can be set within certain limits during production), the resistance is high. If the temperature exceeds a predetermined temperature, the sensor resistance abruptly decreases by several orders of magnitude. Such a change in resistance can be electronically detected by a monitoring device. If hot air emerges from the pipeworks system through a leak, the hot air heats the surrounding sensors until the sensors exceed the predetermined response temperature. The monitoring system detects a leak and responds to the abrupt change in resistance. Additional electronics in the monitoring system is able to control elements that interrupt the air supply in the leaking section by for example closing a shutoff valve.
A further know prior art arrangement uses a thermal switch connected in series with the shutoff valve and also includes a monitoring control device. The thermal switch interrupts the shutoff circuit whenever a predetermined limit temperature within the bleed air duct has been exceeded. The thermal switch prevents possible overheating relating to the temperature within the bleed duct.
Thus, a variety of different thermostat designs have been developed and optimized in an attempt to improve bleed leakage detection reliability and low cost. A technical challenge is to attain an acceptable level of reliability. Although the commonly available thermostatic thermal switch is considered to be highly reliable, the ratio between the thermostat failure rate and duct leakage failure rate may still remain significant. Therefore, the probability is not negligible that a thermostat used to detect bleed air duct failure may have latently failed by the time duct leakage occurs. A thermostat latently fails when it has failed but its failure has not yet been detected.
It would be desirable to provide a bleed air leakage detection approach that provides highly reliable thermostat implementations and yet also minimizes latent thermostatic failures.
The exemplary illustrative non-limiting technology herein provides a bleed leakage detection system including an arrangement of series-connected, disparately placed thermostats. The bleed leakage detection system is capable of detecting the exact place where the bleed air leakage is occurring (e.g., the precise failed junction in bleed air duct work) by detecting which thermostat has opened. Such detection can be accomplished by monitoring voltage or current on a line that connects the thermostats in series with respective resistances. The exemplary illustrative non-limiting implementation also provides a sequential bleed leakage detection system thermostat self-test function (“Initiated Built In Test”—“IBIT”) which allows continuous monitoring of thermostat sensor wiring during flight. The pilot is alerted when the bleed leakage detection system has failed so that appropriate countermeasures and maintenance may be performed.